SCHEMATIC 3. Thanks to the PIC18F2620’s superb
I/O capabilities, a couple of lines of code are all it
takes to replace the CD4069UB inverter, a crystal,
and the capacitor pair supporting the crystal.

CD4069UB
U2F

13
12

CD4069UB

Each AX- 12+ attached to the common link can draw up to
900 mA. So, we need to make sure we provide a + 9. 6 volt
power source that is hefty enough to support every AX- 12+
in the daisy chain.

If you’re wondering what happened to the
PIC18F2620’s crystal, it is not necessary in this design as we
will be coding in the internal 32 MHz clock. We can
conserve I/O pins by building up the design you see in
Schematic 2. If I/O will be plentiful in your design and you
want to eliminate the CD4069, you can. Take a look at
Schematic 3. We have simply given direct control of the
74HC125 OE (Output Enable) pins to the PIC18F2620. The
only caveat in this design is that you must make sure that
you switch the PIC18F2620’s MODE_TX and MODE_RX I/O
lines correctly in the firmware. As you can see in Photo 2,
I’ve gone with the hardware-heavy Schematic 2 design. If
you decide to go with the Schematic 3 design, we’ll code in
and comment out the necessary mode switch code in the
AX- 12+ driver firmware.

The TX and RX LEDs take advantage of the PIC18F2620
EUSART’s logically high idle state. The LEDs will blink with
every passing logic low on the communications link. Thus,
you‘ll see every START bit and every binary zero in the data
stream in the lights.

There’s no rocket science in the power supply or the

SCREENSHOT 1. I recommend adding this little utility to your
programming arsenal. Although it looks like the original site is
gone, search the web using PicMultiCalc and you’ll find archives
that will allow you to download the executable.